Technical Field
The present invention relates to a terminal and a communication method thereof.
Description of the Related Art
When there is a data signal on an uplink of 3GPP LTE (3rd Generation Partnership Project Long Term Evolution), the data signal and control information are time-multiplexed and transmitted using PUSCH (Physical Uplink Shared CHannel) to maintain low CM (Cubic Metric). This control information includes a response signal (acknowledgment/negative acknowledgment (ACK/NACK)) and channel quality (Channel Quality Indicator, hereinafter, referred to as “CQI”).
Different assignment methods are employed for these ACK/NACK and CQI (e.g., see Non-Patent Literatures 1 and 2). To be more specific, some data signals (4 symbols) mapped to resources adjacent to pilot signals (Reference Signal, RS) are punctured and ACK/NACK signals are thereby arranged in some of the resources. On the other hand, CQI is arranged over an entire subframe (2 slots). At this time, since data signals are arranged in resources other than resources in which CQI is arranged, the data signals are never punctured by CQI (see FIG. 1). This is because whether or not ACK/NACK is assigned is determined according to the presence or absence of downlink data signals. That is, since it is more difficult to predict the occurrence of ACK/NACK than predict the occurrence of CQI, puncturing that allows resources to be assigned even when ACK/NACK occurs suddenly is used when mapping ACK/NACK. On the other hand, in the case of CQI, since transmission timing (subframe) is determined beforehand by report information, it is possible to determine resources of data signals and CQI. Since ACK/NACK is important information, ACK/NACK is assigned to symbols close to pilot signals whose channel estimation accuracy is high. This makes it possible to reduce ACK/NACK errors.
Here, MCS (Modulation and Coding Rate Scheme) corresponding to uplink data signals is determined by the base station based on uplink channel quality. Furthermore, MCS of uplink control information is determined by adding an offset to MCS of data signals. To be more specific, since control information is information more important than data signals, MCS of a lower transmission rate than that of MCS of data signals is set for MCS of control information. This allows control information to be transmitted with high quality.
Furthermore, standardization of 3GPP LTE-Advanced which realizes faster communication than 3GPP LTE has been started. The 3GPP LTE-Advanced system (hereinafter may also be referred to as “LTE-A system”) follows the 3GPP LTE system (hereinafter may also be referred to as “LTE system”). 3GPP LTE-Advanced is expected to introduce base stations and terminals communicable at a wide band frequency of 40 MHz or higher to realize a downlink transmission rate of a maximum of 1 Gbps.
Studies are being carried out on the support of SU (Single User)-MIMO communication on LTE-Advanced uplinks. In SU-MIMO communication, a data signal is generated with a plurality of codewords (CWs) and CWs are transmitted in different layers. For example, CW#0 is transmitted in layer #0 and CW#1 is transmitted in layer #1. Here, “codeword” can be interpreted as a unit of retransmitting a data signal. On the other hand, “layer” is synonymous to “stream.”
Furthermore, studies are being carried out on “Layer Shifting” that changes a layer of each CW for every slot (or symbol) to average channel quality of each CW in LTE-Advanced (see FIG. 2). For example, in slot #0, CW#0 is transmitted in layer #0 and CW#1 is transmitted in layer #1. On the other hand, in slot #1, CW#0 is transmitted in layer #1 and CW#1 is transmitted in layer #0. Thus, effects of space diversity are obtained in CW#0 and CW#1.
LTE-Advanced downlinks support carrier aggregation that uses a plurality of downlink unit bands (CC: Component Carrier) for data transmission. When this carrier aggregation scheme is used, A/N is generated for a downlink data signal of each CC. Therefore, A/N needs to be transmitted for a plurality of CCs on uplinks.